The present invention relates to a method and apparatus for monitoring a transmission fibre for damage, and in particular, one in which a loss of signal between two adjacent amplifiers in a transmission path leads to an automatic shut down of the amplifiers.
Optical networks typically employ a number of optical fibre amplifiers to restore optical signal levels at intervals along a transmission path. As the requirement for optical signal power increases, for example to cope with the ever increasing demand for additional channels and/or long haul communication links, so the output power of optical lasers must increase. This presents a potential safety hazard in the event of a fibre break since individuals could then be exposed to harmful levels of radiation.
Recommendations are being developed that define how a network operator must respond to automatically power down line amplifiers. In one such ITU Recommendation (G.664), optical amplifiers are automatically shut down in the absence of an optical signal at an optical input of the amplifier, which would occur in the event of a fibre break. Recommendation G.664 proposes that systems use a form of loss of signal monitoring which is responsive to shut down an amplifier when a traffic signal is lost at the amplifier input. This means that to shut down the two amplifiers nearest to the break, the loss of signal must propagate completely around a loop provided by a pair of optical fibres between two remote terminals before the amplifier up-stream of the break is shut down. This can take several seconds and the delay presents a potential safety hazard. The delay can be particularly long for long haul links. In addition, faults which develop in a fibre can cause an effect termed a xe2x80x9cfibre fusexe2x80x9d in which the fibre core reaches a very high temperature. Since fibre fuse damage can travel at tens of meters per second any delay in shutting down the amplifier can lead to the destruction of long lengths of optical fibre. Should the fibre fuse reach an optical amplifier a great deal of damage can be done to the device.
According to a first aspect of the present invention, a method of monitoring a transmission fibre includes the steps of:
transmitting a monitor signal on the transmission fibre in a direction opposite to the propagation of traffic signals on the transmission fibre;
at an optical amplifier connected to the transmission fibre, detecting the monitor signal; and,
automatically shutting down the optical amplifier in response to a predetermined change in the detected monitor signal.
According to a second aspect of the present invention, an optical amplifier comprises a signal monitoring circuit coupled to an output of the optical amplifier to detect a backward propagating monitor signal, the signal monitoring circuit having a shut down circuit which automatically shuts down the optical amplifier in response to a predetermined change in the backward propagating monitor signal detected by the signal monitoring circuit.
The present invention facilitates a faster shut down of amplifiers than is currently available because it shuts down the amplifier feeding directly into the area of broken fibre first rather than last. This is important for safety reasons but, in addition, in the case of the high power levels expected in future systems, it will limit the damage caused by fibre fuses, a symptom of high power levels in optical fibres. The faster the amplifier shut down, the less damage is caused to the transmission fibre, reducing repair costs, and potentially saving equipment connected to the fibre. Since the fibre fuse damage track can travel at tens of meters per second any improvement in response time will limit the amount of damage caused.
The monitor signal may be intermittent or continuous, and preferably it is the absence of a detected monitor signal that triggers an automatic shut down of the optical amplifier.
Preferably, the monitor signal is generated by an optical source at an optical amplifier, the optical source being coupled for backward transmission of light on the transmission fibre.
More preferably, the monitor signal is Raman pump radiation generated at an optical amplifier. The Raman pump is launched backwards up the transmission fibre to provide Raman amplification within the transmission fibre and some will reach the preceding optical amplifier. The preceding optical amplifier monitors this backwards traveling Raman pump radiation and looks for a loss in this signal. If a loss is detected the amplifier shuts itself down, making the transmission fibre safe, and in the case of a fibre fuse, limiting the amount of fibre that needs to be replaced.
In the preferred embodiment, the output of the optical amplifier is fitted with a tap coupler and a monitoring photodiode connected to a shut down on the amplifier. The amplifier also has a distributed Raman pump which is coupled to counter pump the transmission fibre to provide Raman amplification within the transmission fibre between optical amplifiers.